The present invention relates to a transition between a waveguide channel and a transmission line.
It Is well known in the prior art that electrical signals may be delivered through a variety of conductive media, such as solder traces, electrical wiring, coaxial or triaxial cables, waveguide channels, and microstrip lines, among numerous others. Usually, a given conductive medium will lend itself to a certain application, e.g. microcircuitry is better facilitated through the use of microstrip traces rather than triaxial cables.
Often, a particular electrical application will require that an electrical signal transition between two or more types of conductive media. High-frequency testing of a silicon wafer serves as an effective illustration of this point. Such testing typically involves the interconnection of manufactured testing equipment with an electrical probe, the combination measuring voltages and/or currents at preselected nodes in the device-under-test (DUT) in response to a specific test signal.
Wafer testing equipment is designed to be used repeatedly with a variety of test assemblies, and therefore includes input and output ports by which a particular probe system may be connected. Because coaxial adapters until recently have been unable to efficiently deliver signals above 65 GHz, frequently required for testing of today""s high-speed semiconductor wafers, standard wafer testing equipment traditionally had been manufactured with ports that connect to waveguide channels, which are capable of delivering signals above 65 GHz.
Probes, however, usually deliver the test signal to the DUT through either slender needles or contacts formed on a membrane that overlays the DUT. In addition, most wafer probe assemblies require shielding of the test signal to reduce undesired electrical coupling that may interfere with the test measurements. Accordingly, it is not uncommon for a probe assembly to allow a test signal to first transition from a waveguide to a coaxial line, then to a trace line that terminates at either a needle or a contact depending on the type of probe employed.
Providing an efficient transition between a waveguide and a transmission line has proven problematic. For convenience, these types of transitions will be referred to as waveguide transitions. One widely used waveguide transition employs a waveguide channel into which the tip portion of a transmission line, such as the center pin of a coaxial cable, is inserted at a right angle to one of the interior surfaces of the waveguide. A backshort having a reflective face is also inserted into the waveguide. The backshort is typically made of brass and is oriented perpendicular to the waveguide channel so as to reflect the high-frequency signal towards the transmission line. The backshort is preferably located as close as possible to the transmission line. If properly positioned, the backshort will reflect the alternating signal within the waveguide into a standing wave pattern so that the signal will be induced in the transmission line with minimal degradation.
The waveguide transition just described has a number of limitations. Because a waveguide channel cannot effectively transmit a DC signal, such a transition would be unable to deliver a high frequency signal together with a DC offset, required for example, to hold transistors in an active state during testing. Further, tuning of the waveguide transition is often difficult. Minimum signal transfer occurs when the backshort is spaced apart from the transmission line an integral multiple of one-half signal-wavelengths, while maximum signal transfer occurs at odd multiples of one-quarter signal-wavelengths. Thus at high frequencies, very small deviations from an optimal backshort position may lead to significant losses in signal transfer.
An effective waveguide transition that may retain a DC offset is called a bias tee. Bias tees are used in a number of electrical configurations, including wafer probes. A bias tee typically includes a waveguide transition as previously described where the transmission line is a coaxial cable. A bias tee also includes a connection to a DC source that may provide a bias offset when desired. Any DC offset is combined with the alternating signal present within the waveguide channel by wiring the DC signal from the source to the center pin of the coaxial cable. Usually the DC signal is first passed through a choke so that any high-frequency signals induced in the coaxial cable by the waveguide are isolated from the DC source.
Solutions to the difficulty encountered in tuning the waveguide transition are more problematical. With bias tees, current practice is to adjust the position of the backshort by hand. Traditionally, a backshort is constructed with a necked-down portion having low tensile strength that can be used as a handle. Conductive epoxy is applied around the perimeter of the backshort, which is then inserted into the waveguide channel. Adjustment of the backshort position within the waveguide channel is accomplished manually. Once the desired location of the backshort is obtained, the epoxy is cured by placing the bias tee in a heater. The handle is broken off and removed from the backshort.
This accepted technique has a number of limitations. First, manual adjustment of the backshort does not permit effective fine-tuning, which becomes increasingly difficult at millimeter wavelengths where slight deviations in the backshort position can dramatically decrease performance. Second, if the backshort moves too far within the waveguide, bias circuit components can be damaged. Third, the backshort may shift during the curing process and the epoxy can seep into the waveguide channel which decreases performance. Fourth, once the backshort position is fixed, it is not suitable for a different test frequency range.
In applications other than bias tees, a number of waveguide transitions have been developed that employ adjustable backshorts. Grote et al., U.S. Pat. No. 5,126,696 for example, disclose a W-Band waveguide variable oscillator having a brass backshort equipped with a locking screw. When the locking screw is released, the backshort may be moved manually, thereby adjusting the power output of the oscillator. Similarly, Simonutti, U.S. Pat. No. 4,835,495, discloses a sliding backshort that relies upon friction between the backshort and the surrounding waveguide to maintain the backshort in position unless the friction is overcome by hand pressure. Though these configurations allow the transition to be re-tuned to suit a variety of frequencies, in each of these mechanisms tuning of the backshort occurs by hand, with all of the attendant shortfalls discussed earlier.
What is desired, therefore, is a waveguide transition having an adjustable backshort mechanism in which the backshort may be precisely positioned for maximum efficiency, without significant risk of overtravel and the attendant damage to circuit components. What is further desired is a waveguide transition with an adjustable backshort mechanism that, once adjusted, may be held in place without using conductive epoxy or a similar locking material within the waveguide channel.